Arrangements of the initially named kind are for example always used when it must be made certain before the starting of operations at a high voltage conductor or in its immediate proximity that the conductor is free of voltage or can be earthed without danger for the conducting-away of coupled-in reactive voltages.
For the detection of the alternating voltage and/or the alternating current in overland lines, an arrangement of the initially named kind is known from U.S. Pat. No. 4,002,975 and comprises a measuring head which is arranged at the upper end of a carrier column consisting of insulating material and firmly connected by way of this column with the measuring device. For the measurement of the voltage, the measuring device is so set up on the soil beside a mast of the overland line that the measuring head is disposed in the proximity of the high voltage conductor, however without touching this. Arranged in the measuring head is a sensor, onto which a light bundle is projected in the interior of the carrier column from the measuring device, which light bundle passes through the sensor and is reflected by way of a mirror arranged behind the sensor back again to the measuring device. The sensor in the present case influences the light bundle in the manner that it turns the plane of polarisation of the linearly polarised light in dependence on the intensity of the surrounding electric or magnetic field. The change in polarisation of the reflected light bundle is measured in order to indicate the intensity of the field surrounding the high voltage conductor. Since the polarisation modulation co-efficient of crystals of that kind depends to very strong degree on the surrounding temperature, the measurement head furthermore contains a temperature-measuring device, which is likewise interrogatable by way of a light bundle and either delivers a correction signal or serves to heat the measuring head by way of the carrier column and thus keep it at a predetermined temperature.
This known arrangement is on the one hand unwieldy, because the carrier column, which connects the measuring device with the measuring head, must display an appreciable length in order that the sensor in the case of overland lines can be brought sufficiently closely to the voltage-conducting conductors. For the other thing, the accuracy of the measurement is influenced to an appreciable degree by the spacing, which the measuring head and thereby the sensor possess from the high voltage conductor. Since the spacing of the conductors of overland lines to the soil is defined exactly only in immediate proximity of the carrier masts, this known measuring device must also be set up beside these carrier masts. There, the field surrounding the high voltage conductor is however distorted to a great extent due to the earthed parts of the mast disposed in the proximity so that even when the spacing of the sensor from the high voltage conductor is known to great accuracy, which however is normally not the case in practice, an exact measurement is not possible.
Such a low measurement accuracy however represents an appreciable source of danger, because it does not permit the reactive voltages already mentioned above to be distinguished with absolute certainty from the high voltage which is present at the high voltage conductor when this is connected with the power source.
The extent, to which the employed sensors can vary the polarisation of the light bundle radiated in, is very small even in the presence of the full nominal high voltage so that only a very small measurement signal arises. Beyond that, the measurement light is very strongly damped through these sensors and the associated polarisation filters, whereby only a very weak reflected light bundle, which is difficult to evaluate because of its unfavourable signal-noise ratio, comes back to the actual measuring device in the case of greater measurement paths. For this reason and because of the then required, very long carrier column for the measuring head, the known device is not suitable for a measurement over greater paths, for example at free lines with a height of 100 meters or more above the soil.
The known device is completely unsuitable also for a further important case of application, in which the freedom from voltage of a high voltage conductor disposed in the interior of a switch box filled with SF.sub.6 must be ascertained. Such a switch box may only be opened when no high voltage is present, because the protective effect of the SF.sub.6 is reduced or completely cancelled through the ingress of air and it would come to extremely dangerous and destructive voltage flash-overs, if the switch box were to be opened with high voltage present.
In order to avoid a deterioration of the accuracy of the measuring result, it is further known, to arrange a sensor unit in the close proximity of the respective high voltage conductor, which sensor unit comprises two electric field conductors which are arranged spaced apart and electrically isolated from each other in such a manner that the electric high voltage field generates a voltage drop therebetween. For example, one of these field conductors is the high voltage conductor itself, whereas the other one is a not earthed electrode extending substantially along an equipotential surface of the electric high voltage field. The sensor is electrically connected with said two electric field conductors. By these measures the voltage drop between the two electric field conductors, which is detected by the sensor and translated into an influencing of the light bundle radiated in, is defined exactly, because it now depends only on the fixed mutual arrangement of parts of the sensor unit, which in its turn at least during the measurement due to the direct contact of the high voltage conductor assumes an exactly defined and reproducible position relative thereto.
In all the cases in which the sensor unit is not readily accessible for a checking of its functional capability, an indication, that no high voltage is present at the high voltage conductor, delivered by the measuring device, is reliable only when it is certain at the same time that the sensor unit operates unobjectionably.
However, none of the known devices comprises any means which make possible to check whether the sensor in an inaccessible sensor unit is working correctly or not.